Structural Defects In A TO-220
Ultrasound and X-ray work together to image and evaluate harmless anomalies and potential field failures in a TO-220 package
by Tom Adams, consultant, Nordson SONOSCAN
Launched by an ultrasonic transducer, a pulse of ultrasound travels through the plastic package of an electronic component at a speed around 3,000 m/s. If it strikes a material interface, there are two possibilities: a) if the interface is be-tween two solids, a portion of the pulse will be reflected back to the transducer and another portion will cross the interface; or b) if the second material is air, the pulse is almost entirely reflected, and none crosses the interface.
The second possibility is useful because most of the internal structural anoma-lies and future field failures in a component result from cracks, delaminations, voids or other gaps containing air. A few defects that are imaged do not in-volve air - a tilted die, for example, or a missing solder bump, but both types of interfaces can be located and analyzed by ultrasound.
A TO-220 device like the one discussed in this article is designed to be mount-ed by its tab onto a heatsink in order to dissipate large amounts of heat. Inter-nally, wires run from the die, which is mounted on the substrate, upward to the electrical lead posts. The purpose of acoustic micro imaging of this TO-220, carried out by a C-SAM® tool from Nordson SONOSCAN, was to evaluate interfaces for structural defects: the die attach, the bonding of the wires to the die, and the bonding of the wires to the top of the post. A question that could not be resolved by ultrasound was handled by an X-ray tool.
For acoustic imaging, the TO-220 was first flipped over to image the die attach through the metal back side tab, then turned upright to pulse ultrasound through the encapsulate to image first the die face and second, at a higher plane, the top of the post to which the wires were attached.
At each site, the transducer scanned back and forth a few mm above the area of interest, each second launching thousands of ultrasonic pulses and receiving their return echoes. The echo from each x-y scanned location becomes one pixel in the acoustic image of that area.
From each returning echo the following information is gathered: the echo's acoustic frequency, its amplitude, its polarity, and the depth from which it was reflected. In these images, the colors report amplitude and polarity.
Acoustic image made through the mold compound and showing the die at top and the two posts
An acoustic image of all three elements in the package is shown in Figure 1. The vertical distance between the die and the posts is too great to be within the same focus. Figure 1 is therefore made from parts of two acoustic images, one focused on each of the two depths within the package.
The right lead attached to the die at top has a very small red area, but the wire bond area is essentially intact. The red area at the lower left corner of the die, however, is a void that could grow with repeated thermal cycles and cause wire bonds to break. Whether this void is enough to cause rejection of the part de-pends on the application. It might be suitable in a commercial application, but perhaps not in a military application.
The two wires visible on the die clearly run to the post at the right. There is no indication in this view of wires running to the left post. In this case the wire has a very fine diameter and is too small to be resolved at the low ultrasonic frequency needed to penetrate the mold compound. There is, however, a faintly brighter area on the left post that somewhat resembles a bonded wire.